Color television demodulation system



June 30, 1970 F. H. HILBERT ETAL COLOR TELEVISION DEMODULATION SYSTEMOriginal Filed Feb. 12. 1968 2 Sheets-Sheet 1 Z E FIG! souuo svsrzm -81Y 8C 3| 1 358Mc| DEN;;)B.S;1E; -1 VIDEO RESPONSE [20B 7 2 :5: 0E1.connscron} DEMOD- 1 .L j L, 1 :7 {/20A 20c 1 22 A.G.C. l 1Q 40 f DEMOD.AMF.

SWEEP J cm. I 200; l 42 4 I DEMOD 8 HM 46 48 I i R F 1 L -L +1 05c.sounce F IG. 4 I02 :04 20 cm 20s AMP FiLTER PHASE 5 ESCI Y ESCZ FIGZfreq. i ER 3 338 MHz 3MHZ 3MHz 3.58 7

DEMOD FIG?) 1 Egg FILTER Z so 63 INVENTORS FRANCIS H. HILBERT NORMAN W.PARKER BY mwp, M ,z/VW

ATTORNEYS.

June 30, 1970 F. H. HILBERT ET AL 26,925

COLOR TELEVISION DEMODULATION SYSTEM Original Filed Feb. 12, 1968 2Sheets-Sheet :1

I028 58 FIGS f u 3 E Esc r 205 FILTER 22 :11 I

| I .L 4 g I B V 200 AMP HO I FILTER I 7; 7 I N I rosc 46 FILTER 24 J Al I L I c? EMITTER FOLLOWER L 132 PHASE EouALIzeR FILTER I46 4m:

I48 I FILTER" am:

I I I I I'rs' B.P. FILTER I Esc f M FILTER FILTER a::-- G

DELAY T 456 F f 4 INVENTORS.

4 2 FRANCIS H. HILBERT NORMAN W. PARKER MP 1 I BYWMZZI, 4%? flATTORNEYS.

United States Patent Oflice Re. 26,925 Reissued June 30, 1970 26,925COLOR TELEVISION DEMODULATION SYSTEM Francis H. Hilbert, River Grove,and Norman W. Parker, Wheaten, Ill., assignors to Motorola, Inc.,Franklin Park, Ill., a corporation of Illinois Original No. 3,405,231,dated Oct. 8, 1968, Ser. No. 704,659, Feb. 12, 1968, which is acontinuation-in-part of application Ser. No. 504,523, Oct. 24, 1965.Application for reissue Mar. 10, 1969, Ser. No. 822,079

Int. Cl. H04n 9/50 US. Cl. 1785.4 12 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE A direct demodulator for a composite colortelevision signal decodes the combined video frequency brightnesscomponents and the chroma subcarrier. The demodulator output comprises asignal representing brightness, hue and saturation of the televisionimage. Since the brightness components also beat with the demodulatorcontrol or reference signal of the subcarrier frequency, therebyproducing a spurious signal accompanying the desired signal, a secondarydemodulator section is used to provide a cancellation signal at theoutput of the demodulator in order to eliminate the spurious signal.

CROSS REFERENCE This application is a continuation-in-part of ourapplication Ser. No. 504,523 filed Oct. 24, 1965 and now abandoned.

BACKGROUND The presently used color television signal is comprised ofbrightness or luminance components in a frequency range from zero toseveral megacycles, and a frequency and amplitude modulated subcarrierat approximately 3.58 mI-lz., which represents the hue and saturation ofthe image, i.e., color less brightness." The brightness components andthe modulated subcarrier overlap in frequency to restrict the bandwidthand conserve spectrum use. Interference between these signal componentsis limited because each is comprised of energy bunches which areinterleaved in the spectrum in accordance with known principles.

In many instances the chroma subcarrier is hand selected andsynchronously demodulated at three different phases to produce colordifference signals (RY, B-Y and GY) which are subsequently combined withthe brightness signal [Y] (Y) to produce color representative signals[R, B and G] (R, B and G) for reproducing a composite image in thepicture tube. In some color television systems a composite signal isapplied directly to a demodulator so that a color representative signalis produced without a matrixing operation, but these systems have notbeen altogether satisfactory because a spurious signal is developed dueto the close spectrum relationship of the brightness and subcarriercomponents so that the [produced] reproduced television image isimpaired by a spurious pattern.

An object of this invention is to obviate the production of a spuriousmodulation component in such a direct color signal demodulator.

Another object is to simplify the circuitry for a color signaldemodulator which directly produces red, blue and green representativesignals.

A further object is to provide improved semiconductor circuitry fordemodulating a color television signal of the NTSC type.

SUMMARY The demodulation system of this invention is a direct colorsignal decoder which phase detects a color subcarrier signal along withthe associated brightness signal. The subcarrier signal is modulated atdifferent phase angles to represent saturation of a particular color ofthe image. Three direct, primary demodulators produce red, blue andgreen representative signals including the associated brightnessinformation so that these signals can be coupled to the picture tube forreproduction. Since the brightness signal components extend in frequencyup to and generally into the frequency range of the subcarriermodulation hand, these components modulate with the subcarrier referencesignal to produce a lower modulation sideband of the reference signalwhich falls in the frequency range from the color subcarrier down intothe range of the color representative signals. Such spurious signals arereduced through the use of a secondary demodulator which modulates thebrightness signal components with a reverse phase form of the referencesignal thereby forming a spurious signal cancellation signal which iscoupled to the output of the primary demodulator circuitry. In oneparticular form this can be implemented by using a balanced demodulatorto which the brightness components are applied in the same phase and thesubcarrier components at controlled amplitudes are applied in oppositephase for direct demodulation without spurious signal components.

THE DRAWING FIG. 1 is a block diagram of a color television receiver forexplaining certain aspects of the invention;

FIG. 2 is a series of frequency response curves useful in explainingoperation of the invention;

FIG. 3 is a diagram of a circuit useful in the system of FIG. 1;

FIG. 4 is a block diagram of one form of the invention;

FIG. 5 is a block diagram of another form of the invention;

FIG. 6 is a schematic and block diagram illustrating a modified form ofthe invention; and

FIG. 7 is a schematic and block diagram showing a still further form ofthe invention that may be used in the system of FIG. I.

EMBODIMENTS The color television receiver of FIG. 1 includes tuner andIF amplifier stages 11 which provide a selected and amplified televisionsignal and apply it to the video detector l2. Circuitry 11 also appliesa signal to the sound system 14 for demodulation and amplification ofthe sound subcarrier to drive the loudspeaker 15.

The demodulated signal from the video detector 12 is direct currentcoupled to an amplifier 17 and from there to the demodulation system 20which provides separate red, blue and green representative signals tothe respective amplifiers 22, 24 and 26. These amplifiers areindividually connected to the cathodes of the tri-beam cathode ray tube30 to drive the electron guns in this tube in accordance with knownoperation for production of a composite image in color.

The image reproducer tube 30 includes a plurality of control grids whichare connected to the arms of potentiometers 31, 32, and 33 to provide afixed bias for these grids and control image brightness or beam currentfor the guns of the tube 30.

The signal amplifier 17 is also coupled to an AGC system 40 which isgated to provide a control potential that is variable with the amplitudeof a received signal in order to adjust the gain of the stages incircuitry 11 and maintain a relatively constant amplitude of the signalderived in the video detector 12. Amplifier 17 also feeds the sweep ordeflection circuitry 42 which is coupled to the deflection yoke 44 toprovide suitable sawtooth scanning current for deflection of the beamsof the tube 30. The sweep circuit 42 also generates a suitable highvoltage for the screen of the tube 30 in accordance with known practice.

Amplifier 17 may also supply a control signal to the referenceoscillator 46 in order to generate an accurately phase controlledreference signal for demodulation of the suppressed subcarrier which ismodulated with chrominance information in the composite televisionsignal. In accordance with usual practice, the synchronizing pulses inthe television signal which are utilized to control the sweep circuitry42, are also accompanied by short bursts of control signals atapproximately 3.58 mHz. to be used for synchronization of the oscillator46. Three different phases of the oscillator signal are produced at theoutput terminals 48, 49 and 50. The exact phase angles of the signals isdetermined by several different variables within the receiver itself,such as the dominant color of emission of the various phosphors in thescreen of tube 30. As an example the signal at terminal 48 may be atapproximately 240 with respect to the blue color difference signal, thesignal at terminal 49 may be at approximately and the signal at terminal50 may be at approximately 97 with respect to the blue color differencesignal.

A signal applied to the input circuit A of the demodulator isillustrated in the response vs. frequency curve of the F16. 2A. Thevideo frequency brightness cornponents E extend from zero to over 2 mHz.and sometimes as high as 3 or 4 mHz. The subcarrier wave representingchrominance or color difference information is modulated atapproximately 3.58 mHz. with the component of one phase E in a band ofapproximately .5 mHz., and in another phase as a vestigial sidebandsignal E with the lower sideband extending below 3 mHz. Present dayreceivers generally use a subcarrier bandwidth of approximately .5 mHz.total for all subcarrier components and the following description willassume such operation. However it will be apparent that the principlesdiscussed are equally applicable to deriving color components at variousbandwidths and producing the color signals directly from them.

it should further be noted that the frequency response characteristicsof the various parts of the receiver can be modified and correlated inaccordance with known receiver design to establish an overall desiredvideo response. This correlation within the receiver need not bediscussed to understand the present invention.

The NTSC signal has video frequency brightness components in thefollowing relationship in order to improve compatability for receptionby a monochrome receiver so that its grey scale of a color image will bein accord with the visual response of the colors by the human eye. Thebrightness signal is as follows:

In this formula E represents a signal voltage of a luminance componentof a picture element and E E and E are respectively the signal voltagesrepresenting the colors red, green and blue for that picture element.The chrominance modulated subcarrier wave is represented as follows:

E K (E E COS wt+K2(E Ey) sin wt Cit In terms of the green chrominancecomponent the subcarrier can be represented as:

In the above formulas K equals l/l.l4; K equals 1/2203; K equals l/0.70;and K equals As is understood in the television art the signals areconstituted for proper compatibility for monochrome reception of thesignal and production of a grey scale matching the color response of theeye, as well as for a limited amplitude swing of the quadraturemodulated subcarrier wave with respect to By in order to limit the sizeof the composite signal (E :E +E for proper handling in a receiver.

FIG. 3 represents a direct color signal demodulator. The compositesignal, including the brightness components and the subcarrier wave, isapplied push-pull to the demodulator 20B which is controlled inconduction by a reference signal of the subcarrier frequency of properphase. For example the signal from terminal of oscillator 46 can be usedto demodulate for the red representative signal. The output of thedemodulator is applied to a video frequency filter 58 to establish a lowpass range up to, for example, 3 mHz. to be applied to the amplifier 22of FIG. 1. The pass range of filter 58 is represented as frequency band58' in FIG. 2C. The demodulator of FIG. 3 is of the balanced type butpartially unbalanced to compensate for the luminance to subcarrier ratioof the composite signal. This unbalance effectively compensates for thedemodulator efficiency and the K factors in the equations of the signal.

The circuit of FIG. 3 has input capacitors 59 and 60 to which oppositephases of the composite signal are applied with respect to ground.Capacitor 60 is connected to the arm of variable resistor 61. having afixed terminal connected to ground and another fixed terminal connectedto one of the input terminals of the detector circuit. A diode 62 has ananode connected to capacitor 59 and diode 63 has a cathode connected tocapacitor 60. The cathode and anode of diodes 62 and 63 are coupledtogether to the series combination of capacitors 65, 66, theinterconnection of which is coupled to the terminal 50 of the oscillator46.

The output of the demodulator 20B is derived at the interconnection ofthe resistors 68, 69 which are series connected across capacitors 65,66. This output is applied to the filter 58 to define the videopassband.

The operation of the demodulator 208' in FIG. 3 includes the alternateconduction of the diodes 62, 63 by opposite phases of the colorreference signal applied from oscillator 46. Opposite phases of thechrominance modulation components are thus conducted by the diodes todevelop a potential at the junction of resistors 68, 69 which representsboth the amplitude of the chrominance modulated subcarrier at the phaserepresenting the red signal information in addition to the associatedluminance information. Circuit 2118' is unbalanced to a selected degreeby adjustment of the variable resistor 61 so that it will conduct theproper amplitude of luminance components to be combined with thedemodulated chrominance information for the direct production of the redrepresentative signal. That is resistor 61 is adjusted so that theluminance components are not balanced out through equal and oppositeconduction of the diodes 62 and 63.

An understanding of the functioning of the circuit of FIG. 3 can be hadby considering the reference signal as a rectangular gating signalcausing the demodulator to switch or sample the applied composite inputsignal by electron control means (cg. a diode) which is essentiallyeither conductive or nonconductive. Mathematically the operation can beexpressed by multiplying the composite color signal E by a cyclefunction having a value [of] 1 with the electron control means closedand the value zero when the control means is open. Such a gating signalto demodulate for red information can be represented as:

4 G(t) :AT/T(1+2A cos wt +2A2 cos Zwt 2A cos l'lwl) 2n A,,=i 1rf o (t)cos stat,

and AT/T is the duty cycle For demodulating the blue signal the cosineterms become sine terms and for the green signal the cosine angle isw+146.

To develop the red representative signal the composite signal E ismultiplied by the gating function and the product is as follows:

In Equation 5 the first and fifth terms combine (by adjusting theunbalance of demodulator B to control E since K is less than 1 and A;cannot exceed 1) thus producing signal E and E of FIG. 2B. Similardemodulation can take place for the blue and green representativesignals with proper demodulator unbalance.

The second and third terms of the product are the original subcarriercomponents which are removed to the extent they fall outside thepassband of the filter 202 as seen by comparing the passband of FIG. 2Cwith the modulation range E of FIG. 2A, the wider range chrominancemodulation being ignored as previously discussed. The sixth and seventhterms of the demodulation product are at twice the subcarrier frequencyand higher so that they fall outside the video signal bandpass F. Thefourth term represents a modulation product which is a spurious signaldue to beating of the brightness signal E with the first order periodiccomponent of the gating signal. Depending upon the frequency range ofthe brightness signal applied to the demodulator of FIG. 3, the lowersideband of this spurious component S (FIG. 2D) can extend all the wayfrom the frequency of the reference signal (3.58 mHz.) down to zero, andtherefore throughout most or all of the video passband 58'. The higherthe frequency of the brightness component applied to the demodulator thelower in frequency the lower sidebands will extend within the outputrange of filter 58.

Such a spurious signal S may have a substantial amplitude so that itappears as a pulse on the edge of any substantial luminance change inthe reproduced image. Since this spurious signal will be changing inphase with others produced by the blue and green signal demodulatorsthis undesired portion of the image will appear to move along theluminance difference transition in the picture giving the appearance ofa crawling pattern. Thus, the problem is produced when the highestfrequency of the brightness signal Ey is close enough in frequency tothat of the reference signal (here 3.58 mHz.) such that their modulationproduct, or part of it, falls within the low pass range from thedemodulator to the picture tube.

It may be seen that the low pass filter 58 will remove the uppersideband component of the fourth term of the modulation product and thecarrier thereof, but the lower sideband remains. That portion of thebrightness signal E interleaved with the subcarrier E in FIG. 2A can beseparated from the subcarrier by known comb filter techniques. Howeverthose signals may be tolerable in some practical systems and they arenot normally removed in the present day commercial receivers. Inaccordance with teachings hereof the brightness components lower thanthe lowest selected subcarrier sidebands are prevented from generatingcross color interference or spurious signal as described below. If combfilter techniques are used to separate, the brightness and chrominancecomponents the spurious signal elimination as described below can beused for the entire luminance range.

This spurious signal S produced by modulation of the referenceoscillator signal in the demodulator by a luminance step consists of asignal transient or voltage pulse in the video frequency output of thedemodulator and we contemplate cancelling this spurious component by anequal amplitude and opposite phase signal applied at the output of thedemodulator that produced the spurious signal. Such a system is shown inFIG. 4.

In FIG. 4 the amplifier 17 provides the demodulated composite videosignal including the luminance components and the chrominance modulationcomponents to a phase equalizer which is used to compensate for any highfrequency roll-off or other undesired frequency tilt in the compositesignal which may be produced as it is translated through the stages 11.12 in the receiver. The phase equalizer 100 is coupled through a filter102 that passes all of the luminance and chrominance modulationcomponents to a phase splitter 104. The phase splitter 104 appliesopposite phases of the composite video signal to the input terminals ofth primary demodulators 20B, C and D, all with respect to ground.

The signals from the phase equalizer 100 are also applied through afilter to a phase splitter 112. It is noted that the filter 110 passesonly the luminance components to the exclusion of the chrominancemodulation components so that phase splitter 112 couples opposite phasesof the luminance components to demodulator 114. A reference oscillatorsignal from terminal 50 is applied to both the desired signaldemodulator 20B and to the cancelling signal or secondary demodulator114.

As previously described, the output of the demodulator 20B will, due toits unbalance and the modulation of the reference signal by theluminance components, produce a spurious signal, in addition to thedesired color representative signal. All of the signal energy is appliedto the signal adder circuit 116. The demodulator 114, which has appliedto it only demodulated luminance components, produces a counter phasespurious component in its output (C in FIG. 2D) which is also applied tothe adder circuit 116. Rectifiers in the secondary demodulator 114 arepoled in order that its output spurious signal C will be of the correctphase for cancellation of the spurious signal from the demodulator 20Bso that the output of the adder circuit 116 includes a colorrepresentative signal without any spurious luminance component in therange of filter 58. It is contemplated that the modulator 114 could beconstructed in a manner similar to that of FIG. 3, and, it can, ofcourse, be duplicated with other adder circuits associated with thedemodulators 20C and 20D for production of blue and green representativesignals, free of spurious luminance signals.

The operation of the demodulator 114 can be understood mathematically byreference to the following product:

Its operation is the multiplication of the luminance components E; by aphase reversed form of the gating function of Equation 4. The output ofthe demodulator 114 is thus seen to include an E component (not requiredas 20B provides that also) as well as a term of opposite sign to that ofthe fourth term of Equation 5 so that it is cancelled. There are alsohigher order components as harmonics of the subcarrier frequency butthese fall outside the passband 58'.

In the case of demodulators for the blue and green representativesignals, the A coefiicients of Equation 6 would be changedcorrespondingly as previously discussed and the phase of the angle wouldbe changed to w1+ 146 for the green representative signal and the ACcomponent would be changed to sine functions for the blue representativesignal.

In the circuit of FIG. 3 a portion of the composite signal is availablein the demodulator due to the onbalance introduced by resistor 61. Thisof course means that the brightness signal E is available for properdemodulation with the subcarrier wave to directly produce the desiredcolor representative signal. Thus control of the variable resistor 61provides the proper level of the E, signal with respect to the colorsubcarrier to compensate for the demodulator efiiciency and the Kcoefficient of the composite video signal.

Another method of achieving the proper combination of brightness tochrominance is to vary the effective values of the K coefficients byadjusting the amplitude of the subcarrier wave with respect to thebrightness componcnts in the range below the subcarrier components.

The A constants in the demodulation formulas for the composite signalare 1.14 for the red signal, 2.03 for the blue signal. and 0.70 for thegreen signal. Whereas the green representative signal can be demodulatedwithout special techniques, the red and blue representative signalsrequire special handling in order to establish the proper ratio of thebrightness signal to the color component so that the resultant signal ofthe demodulator represents the brightness, hue and saturation forapplication to the picture tube 30, In a system which does not adjust Ean E adjustment is feasible since the demodulating signal is a limitingfactor. The A constants represent the coefficients of a Fourierexpansion of the gating function. If the gate signal is assumed to berectangular (as it may be for practical purposes if. it has sufiicientamplitude. even though it may in total be a sine wave) the coefficientscan be represented as:

where F equals the duty cycle of the gate pulse. Since this function hasa maximum value of unity, a technique must be used to effectivelyachieve a K over one in the signal so the A and K product equals one(see Eq. 5).

FIG. 5 illustrates a circuit which uses step filters for subcarrieramplitude correction, and which shows the signal demodulators as gates.The spurious signal cancellation occurring in FIG. 5 is similar inoperation to that of the circuitry of FIG. 4.

In FIG, 5 the demodulator 20B" is represented as a rotating switch armunder control of the signal from oscillator 46. Its output is applied toa filter 58 having a pass range for example corresponding to that ofFIG. 2f.

Amplifier 17 supplies the composite signal E to the step filter 1028having an increased amplitude for the subcarrier components around 3.58rnHz. The demodulator 20B" is also fed with signals from low pass filter1108 which selects only the brightness component. It can be seen thatthe brightness signal B and the subcarrier E will be sampled by aprimary demodulator portion and applied to the filter 58 to produce thered representative signal. In addition an opposite phase sampling of theBy components takes place in a secondary section from the filter 1108 tocancel the spurious signal modulation components as previouslydiscussed.

The demodulators 20D" and 20C" produce the green and blue representativesignals since these demodulators are conductive at different portions ofthe reference signal cycle corresponding to the color information atthat phase of the reference. The step filter 102D and the step filter102C as well as the filter 102B all sufficiently peak the subcarrier sothat the A constants can be arbitrarily chosen to be less than 1 (forexample by adjusting the sample width). As an example the step filter102D may double the amplitude of the subcarrier over the amplitude ofthe B components, the filter 1028 may increase the subcarrier 3.5 timesand the filter 102C may increase the subcarrier 6.3 times. In this waythe effective values of the K components are changed in the compositesignal applied to the switching devices so that the desired colorrepresentative signals are directly produced. Since the high frequencybrightness response in the region of the subcarrier frequency (where itmay overlap the subcarrier modulation components) will also be undulypeaked by the step filters in the circuit of FIG. 5, it may be desirableto reduce the video frequency response in the region of subcarrierfrequency in the filters at the output of the demodulators 20B", 20D and20C".

In implementing a circuit such as shown in FIG. 5 it will be recognizedthat a pair of diodes can be used to perform the switching function andthat these would be oppositely poled and controlled by a signal of theproper phase angle in the Oscillator 46. The circuit of FIG. 3 usesunbalance of the detector to adjust the brightness to chrominance ratiowhereas the circuit of FIG. 5 uses step filters for chrominance tobrightness ratio but in either case the brightness and reference signalmodulation product is cancelled through a secondary modulation processwhich provides a cancelling signal from combination of the brightnesssignal with a properly phased reference signal.

In the demodulator system of FIG. 6 production of the spurious signalcomponents S (FIG. 2D) is avoided due to application of the chrominancesubcarricr wave in a balanced manner to a balanced demodulator and inthe application of the luminance components in an unbalanced manner tothe balanced demodulator. This has the advantage that the subcarrierwave will be demodulated in a full wave manner so that less subcarricrprepeaking is required as compared to the circuit of FIG. 5. Thefunction of the primary and secondary demodulators is combined so thateach section of the demodulator performs both functions. This operationcan be understood by considering the operation mathematically.

Demodulator 20D has a second portion with opposite phase subcarrier andreverse phase diode (equivalent to opposite phase reference signal) withrespect to the first section of the demodulator. Accordingly thefollowing equation represents the operation of the second section:

When the Outputs of the demodulator sections represented by Equations 5and '7 are combined, the first terms of the two equations are additive,and the second and third terms cancel one another to remove the originalsubcarrier wave in the output. The fourth terms of the equations cancelas is desired to obviate the spurious brightness and referencemodulation component. The fifth terms are additive and the remainingterms are inconsequential in the system. The circuit of FIG. 6 operatesin accordance with the above description.

In FIG, 6 the amplifier 17 applies the demodulated composite videosignal to the phase equalizer and from there the signal is coupled to anamplifier and a phase splitter 130. Amplifier 125 includes a transistor126 having an emitter electrode connected to gr und through a resistor128. A variable arm with resistor 128 is bypassed for signal frequenciesby a capacitor 129 so that at the collector electrode of transistor 126there appears a selected amplitude of the composite video signal.

A time delay and emitter follower 132 couples the composite video signalto the interconnection of resistors 134 and 135 connected between thetransistorized emitter followers 137 and 138.

The phase splitter 130 includes a transistor 140 having a collectorelectrode coupled to a wide band filter 142 which passes the compositesignal including luminance and chrominance modulation in the frequencyrange out to approximately 4 megacycles. The emitter electrode oftransistor 140 is coupled through a filter 144 having a pass range up toapproximately 3 megacycles, which thus passes the luminance componentsand excludes the chrominance modulation components. The Outputs offilters 142, 144 are coupled respectively through resistors 146 and 147to a fixed terminal of variable resistor 148 which has a further fixedterminal connected to ground. The variable arm of the resistor 148 iscoupled to the transistor 151 in the phase splitter 150. The collectorand emitter electrodes both include load impedances, namely resistors153 and 154 respectively which are coupled to the emitter followerstages 137, 138.

Accordingly, opposite phase of differing portions of the composite videosignal are applied to the variable resistor 148. These differentportions include opposite phases of the luminance components so thatthese are effectively cancelled leaving only the chrominance componentscoupled through the filter 142. Thus adjustment of resistor 148 willplace a variable drive of the chrominance modulation components on thebase electrode of transistor 151 so that the output thereof will beopposite phases of a selected amplitude of the chrominance modulationcomponents. The emitter followers 137 and 138 will thus produce outputsignals which are opposite phases of the chrominance modulationcomponents and the same phase of the luminance modulation components,applied to the emitter followers in a parallel circuit through theresistors 134 and 135.

Emitter follower circuits 137 and 138 are each coupled to the two inputterminals of the demodulator circuits such as 201). It may be seen thatthe particular circuitry of demodulator circuit 20D correspondsgenerally to the circuit of FIG. 3, with the exception that the circuit20D is balanced and no unbalancing resistor, such as resistor 61, isincluded. As the input to demodulator 20D will include opposite phasesof the chrominances modulation components, these will be detected asdescribed in connection with the circuit of FIG. 3. Furthermore, theinput to the demodulator 20D will include the same phase of luminancecomponents applied to each input terminal, with respect to ground, sothat the luminance components (E will be conducted into the demodulatorto be combined with the demodulated chrominance component (E -E and acolor representative signal is directly produced at the output.

FIG. 7 illustrates a demodulation system incorporating a form of thesubcarrier peaking suggested in the system of FIG. together withbalanced demodulators which are driven as a two section demodulatorwhich is fed with the luminance components B in push-push and thesubcarrier wave E in push-pull for alternate sampling of the compositein accordance with the Equations 5 and 7 as in 20D of FIG. 6.

In the circuit of FIG. 7 a composite signal from amplifier 17 is appliedto a bandpass filter 170 which selects the subcarrier wave. Amplifier 17is also DC coupled through a compensating delay line 172 to the centertap of the secondary of transformer 175. A selected amplitude of thesubcarrier wave is applied to the primary of the transformer 175 toappear across the secondary with opposite phases with respect to theluminance feed point.

The demodulators 20B" includes a resistor network coupled across thesecondary of transformer 175 with the diodes 177 and 178 series coupledacross intermediate resistor 180 of the resistive network. Since thesignal B; is at the same amplitude and polarity on both sides of thesecondary winding of transformer 175 it will be conducted at the sameamplitude by both diodes. On the other hand opposite phases of thesubcarrier wave are applied to the ends of resistor 180 and selection ofthe values in this resistive network can thus provide a desired ratio ofsubcarrier to luminance effectively producing a step filter like 1023 atthe input to the diodes 177 and 178.

This compensates for the A coefficients previously discussed, as well asthe demodulator efficiency.

The diodes 177 and 178 are oppositely poled and coupled through acapacitor to the terminal 50 of the oscillator 46. These diodes are alsocoupled to the filter 58 which applies the red and high frequencyluminance representative signal corresponding to FIG. 2B to theamplifier 22. The other demodulators 20C and 20D" correspond incircuitry to 20B" except that the phase angle of the applied referencediffers and the input resistive networks differ in order to properlypeak the subcarrier wave with respect to the luminance component. Eachdemodulator is DC coupled through its filter and amplifier to thereproducer 30.

The operation of the circuit of FIG. 7 corresponds to that given forcircuit 20D of FIG. 6. The selected phase of the subcarrier wave will beconducted to the demodulator output filter for effective full waverectification of it and the signal By Will be present in the demodulatorto be processed at twice the reference carrier frequency forsimultaneous combination to produce the color representative signaldirectly. While each one of the diode electron switch devices willproduce a modulation product between the B signal and the appliedswitching sig ha] of reference frequency, the two switches conduct outof phase with one another so that the modulation products of the twocancel in the demodulator output, all as discussed in connection withEquations 5 and 7. Thus direct color signal demodulation takes place butthe false modulation components normally developed in such a system areobviated.

The system hereof therefore provides decoding of the multiplex colortelevision signal of the NTSC type. This system could also operate upona composite signal of intermediate frequency (before demodulation of themain carrier) if proper circuitry is used for subcarrier amplitudeadjustment. In successful practice the system produces the desired red,green and blue representative signals from the composite signal inwhatever form without the need for matrixing techniques and adjustmentsnormally associated with receivers utilizing more than the three videosignal channels.

We claim:

[1. A color television demodulation system for utilizing a compositesignal including video frequency brightness components and a subcarrierwave modulated in amplitude and phase to represent color information,said subcarrier wave having modulation components as least partiallyoverlapping in frequency the brightness components, said demodulationsystem including in combination:

a first synchronous demodulator including an input circuit for thecomposite signal and means for applying thereto a control signal of thesubcarrier frequency, said first demodulator also including an outputcircuit for the demodulated video signal representing brightness, hueand saturation information in the composite signal, the video frequencybrightness components beating in said first demodulator with the controlsignal of subcarrier frequency to produce a spurious signal in saidoutput circuit.

and a second synchronous demodulator including means for applyingthereto the video frequency brightness components and a signal phaselocked to the subcarrier frequency to beat the same together to producea cancellation signal for the spurious signal.

and means for applying the cancellation signal to said output circuit ofsaid first synchronous demodulator with an amplitude and phase to offsetdevelopment of the spurious signal in said first synchronousdemodulator] [2. The demodulation system of claim 1 in which said firstdemodulator is a half-wave rectifier for the subcarrier wave] [3. Thedemodulation system of claim 1 in which said first demodulator is afull-wave rectifier for the subcarrier wave] [4. The demodulator systemof claim 1 which includes means for applying the subcarrier wave to saidsecond demodulator and wherein said second demodulator tends to producea further spurious signal from beating of the brightness components andthe control signal, and wherein the first and second demodulatorsconduct out-ofphase with one another so that said first demodulatorproduces a further cancellation. signal for the further spurious signal][5. The demodulation system of claim 1 in which said first demodulatoris unbalanced for conducting a preselected portion of the brightnesscomponents] 6. The demodulation system of claim [I] 10 wherein saidinput circuit includes means for establishing a selected amplitude ofthe subcarrier wave with respect to the brightness components.

[7. In a color television receiver including receiver c rcuit meansproviding a demodulated color television signal comprising Videofrequency luminance components in a given frequency range, and asubcarrier modulatedan amplitude and phase to represent color differenceinformation and having modulation components overlapping the givenfrequency range, and oscillator means providing an oscillator signal ofthe subcarrier frequency and of selected phase for demodulating onephase of the modulated subcarrier, the combination of a demodulatorcircuit coupled to said receiver circuit means and to said oscillatormeans to be controlled by the oscillator signal and the modulatedsubcarrier and the luminance components, said demodulator circuit havingan output circuit and being operative to detect one phase of themodulated subcarrier in the presence of the luminance components toproduce a color representative signal, and said demodulator circuitfurther being operative to produce spurious frequency components withinthe frequency range of the luminance components in said output circuitthereof by beating of the luminance components with the oscillatorsignal, and means including a further demodulator coupled between saidcircuit means and said output circuit for producing a cancelling signalfor the spurious frequency components appearing in said output cirwit]8. In a color television receiver including receiver circuit meansproviding a demodulated color television signal comprising videofrequency luminance components in a given frequency range, and asubcarrier modulated in amplitude and phase to represent colordillerence information and having modulation components overlapping thegiven frequency range, and oscillator means providing an oscillatorsignal of the subcarrier frequency and of selected phase fordemodulating one phase of the modulated subcarrier, the combination of ademodulator circuit including three phase detectors, each with dualinput circuits for demodulating opposite phases of applied signals,means applying different phases of the oscillator signal to saiddetectors, and means coupling said receiver circuit means to saiddetectors including a network coupling the luminance components to saiddetectors with the same polarity at both input circuits of each andcoupling the modulated subcarrier to said input circuits of each withopposite phases.

[9. In a color television receiver including circuit means providing atelevision signal comprising video frequency luminance components in agiven frequency range and a subcarrier modulated in amplitude and phaseto represent color ditference information, and oscillator circuit meansproviding a control signal of the subcarrier frequency, the combinationof a demodulator circuit including an output circuit and switching meansunder control of the oscillator signal, means applying the colortelevision signal to said switching means so that a signal representingbrightness and saturation for a given color is applied to said outputcircuit, and further switching means coupled to said output circuit andunder control of the oscillator signal to be conductive in oppositephase to said first mentioned switching means, and means applying atleast the luminance components to said further switching means forcancelling luminance components modulated with the oscillator signal insaid output circuit] 10. A color television demodulation system forutilizing a composite signal including video frequency brightnesscomponents and a subcarricr signal modulated in amplitude and phase torepresent polor information, said subcarrier signal having modulationcomponents at least partially overlapping in frequency the brightnessc0mponents, and an oscillator circuit providing an oscillator signal atthe subcarrier frequency and of selected phase for demodulating onephase of the modulated subcarricr, said demodulation system including incombinarion:

a dual section demodulator circuit having a pair of input circuits fordemodulation of opposite phases of applied signals;

means for connecting said oscillator circuit to said demodulator circuitfor controlling the conduction thereof at the opposite phases; and

means for applying the brightness components to both of said inputcircuits at the same phase and for applying the subcarrier signal tosaid input circuits at opposite phases, said demodulator circuitdetecting the selected phase of the modulated sub carrier signal in thepresence of the associated brightness components to produce a colorrepresentative signal, spurious brightness signals being cancelled bythe alternate conduction of the sections of the demodulator circuit.

11. The demodulation system of claim 10 wherein a plurality of dualsection demodulator circuits are provided, each under control of thesignal from the oscillator circuit at a different phase thereof and eachof which includes means for adjustment of the ratio of the brightnesscomponents to the subcarricr signal com poncn ts.

12. The demodulation system of claim 10 wherein the means for applyingthe brightness components to the input circuits further includes afilter for selecting the subcarricr signal and means for adjusting theamplitude thereof independently of the amplitude of the brightnesscomponcnts.

13. A color television demodulation system for utilizing a compositesignal including video frequency brightness components and a subcdrriersignal modulated in amplitude and phase 10 represent color information,said subcarrier signal having modulation components at least partiallyoverlapping in frequency the brightness (0mponenlir, and an oscillatorcircuit providing an oscillator signal at the subcarrier frequency andof selected phase for demodulating one phase of Ihc modulatedsubcarrier, Said demodulation system including in combination:

a dual section demodulator circuit having a pair of solid stateswitching devices for demodulation of applied signals;

means for connecting said oscillator circuit to said switching devicesfor controlling the conduction thereof at opposite phases of theoscillator signal;

an input network having a pair of branches, each branch being of equalimpedance and being connected to a diflerent one of the switchingdevices;

means for applying (he brightness components with the same phase to eachof the branches;

means for applying the subcarrier signal to 1/10 bru/urluxs' in oppositephase across the branches; and

on output circuit coupled in common to said switching devices to providea color representative signal, whereby the demodulator circuit detects aselected phase of the subcnrricr signal in the presence of theassociated brightness components to produce the color representativesignal from alternate conduction of the switching devices, with spuriousbrightness signal components being cancelled by the alternate conductionof the switching devices.

14. A color television demodulation system for utilizing a compositesignal including video frequency brightness components and a subcarriersignal modulated in amplitude and phase to represent color information,said subcarrier signal having modulation components at least partiallyoverlapping in frequency the brightness components, and an oscillatorcircuit providing an oscillator signal at the subcarrier frequency andof selected phase for demodulating one phase of the modulatedsubcarrier, said demodulation system including in combination:

first filter means for passing the brightness and modulated subcarriercomponents;

second filter means for passing only brightness components to theexclusion of the subcarrier signal modulation components;

means for supplying said composite signal to the inputs of the first andsecond filter means;

a first partially-unbalanced dual section demodulator circuit having apair of inputs for demodulation of opposite phases of applied signals;

a second dual section demodulation circuit having a pair of inputcircuits for demodulation of applied signals;

means for connecting said oscillator circuit to said first and seconddemodulator circuits for controlling the conduction thereof at theopposite phases;

means for applying the output of the first filter means in oppositephases to the input circuits of the first demodulator;

means for applying the output of the second filter means at oppositephases to the input circuits of the second demodulator; and

means for applying the output of the second demodulator to the outputcircuit of the first demodulator with an amplitude and phase to ofisetdevelopment of spurious signals in the first demodulator, the amount ofunbalance of the first demodulator operating to adjust the amplitude ofthe brightness components with respect to the demodulated subcarriercomponents for the direct production of a demodulated video signalrepresenting brightness, hue and saturation.

15. The combination according to claim 14 wherein the first filter meansis a low pass filter having an upper cutofl frequency of a firstpredetermined value and wherein the second filter means is a second lowpass filter having an upper cut-off frequency of a second predeterminedvalue which is less than the first predetermined value.

16. A color television demodulation system for utilizing a compositesignal including video frequency brightness components and a subcarriersignal modulated in amplitude and phase to represent color information,said subcarrier signal having modulation components at least partiallyoverlapping in frequency the brightness components, and an oscillatorcircuit providing an oscillator signal at the subcarrier frequency andof selected phase for demodulating one phase of the modulatedsubcarrier, said demodulation system including in combination:

a dual section demodulator circuit having a pair of input circuits fordemodulation of opposite phases of applied signals;

means for connecting said oscillator circuit to said demodulator circuitfor controlling the conduction of the sections thereof at the oppositephases; and

means for applying the brightness components to both of said inputcircuits at the same phase and for applying the subcarrier signal andbrightness components overlapping the subcarrier signal to the itt putcircuits at opposite phases, said demodulator circuit detecting theselected phase of the modulated subcarrier signal in the presence of theassociated brightness components to produce a color representativesignal, spurious brightness signals and said overlapping brightnesscomponents being cancelled by the alternate conduction of the sectionsof the demodulator circuit.

17. The demodulation system of claim 16 wherein the means for applyingthe brightness components to the input circuits further includes meansfor selecting the subcarrier signal and overlapping brightnesscomponents and means for adjusting the amplitude thereof independentlyof the amplitude of the remainder of the brightness components.

18. The demodulation system of claim 16 wherein the means for applyingthe brightness components to the inut circuits further includes meansfor establishing the subcarrier signal and overlapping brightnesscomponents at an amplitude substantially exceeding the amplitude of thebrightness components applied at the same phase to said input circuits,and in which said demodulator circuit is balanced with respect tosignals applied at opposite phases to the input circuits.

19. A color television demodulation system for utilizing a compositesignal including video frequency brightness components and a subcarriersignal modulated in amplitude and phase to represent color information,said subcarrier signal having modulation components at least partiallyoverlapping in frequency the brightness components, and an oscillatorcircuit providing an oscillator signal at the subcarrier frequency andof selected phase for demodulating one phase of the modulatedsubcarrier, said demodulation system including in combination:

a dual section demodulator circuit having input circuits fordemodulation of opposite phases of applied signals;

means for connecting said oscillator circuit to said demodulator circuitfor controlling the conduction thereof at the opposite phases;

means for applying the brightness components to selected input circuitsat the same phase and for applying the subcarrier signal to selectedinput circuits at opposite phases, said demodulator circuit detectingthe selected phase of the modulated subcarrier signal in the presence ofthe associated brightness components to produce a color representativesignal: and

means for combining the outputs of the sections of the demodulatorcircuit so that spurious brightness signals are cancelled by thealternate conduction of the sections of the demodulator circuit.

References Cited The following references, cited by the Examiner are, ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,917,573 12/1959 Holmes 1785.4

ROBERT L. GRIFFIN, Primary Examiner R. MURRAY, Assistant Examiner

